It is known that a cholesteric liquid crystal or a nematic liquid crystal containing a chiral material, forms a cholesteric phase liquid crystal having a twisted alignment of spiral structure, and in a case where a spiral pitch P is equivalent to an wavelength λ of incident light, the liquid crystal has circular polarization dependence (it is referred to as “circular-polarization-selective-reflection”) whereby circularly polarized light having the same rotational direction as the twist direction of a liquid crystal into which the light is incident from the direction of spiral axis is reflected, and circularly polarized light having the opposite rotational direction is transmitted.
Further, in this cholesteric phase liquid crystal, for example, provided that twist direction of the liquid crystal is clockwise, the liquid crystal has a wavelength band (it is referred to as “reflective wavelength band”) providing “circular-polarization-selective-reflection” to incident light of clockwise circular polarization, and the liquid crystal shows a large optical rotatory dispersion (a phenomenon that optical-rotation changes depending on wavelength) in the vicinity of the reflective wavelength band (vicinity of reflective wavelength band). On the other hand, for incident light having a counterclockwise circular polarization, there is no reflective wavelength band and the liquid crystal shows a small optical rotatory dispersion according to a description of Non-Patent Document 1.
Namely, in a transmitting wavelength region in the vicinity of the reflective wavelength region of the cholesteric phase liquid crystal having “circular-polarization-selective-reflection”, function, significant difference arises between optical rotation properties to clockwise circularly polarized light and counterclockwise circularly polarized light.
Further, it is known that in a cholesteric phase liquid crystal having relatively small spiral pitch P, a cholesteric blue phase (a liquid crystal in the state of cholesteric blue phase is referred to as “blue phase liquid crystal”) is developed in an intermediate temperature range between temperature ranges of cholesteric phase and isotropic phase.
The blue phase liquid crystal has a three-dimensional periodical grating structure in which cylindrical portions each having a double-twisted spiral internal structure, are spatially regularly arranged, which causes Bragg diffraction of incident light having a wavelength and an incident angle satisfying diffraction conditions. Diffraction light of Bragg diffraction generated here has a circular polarization dependence in the same manner as a cholesteric phase liquid crystal, but since its reflective wavelength band developing “circular-polarization-selective-reflection” is narrower than that of a cholesteric phase liquid crystal, significant difference arises between optical rotation properties to clockwise circularly polarized light and counterclockwise circularly polarized light in a transmitting wavelength band in the vicinity of the narrower reflective wavelength band.
Further, since the temperature range of conventional cholesteric blue phase has been as narrow as a few ° C., it has been difficult to realize practical elements as applications of a blue phase liquid crystal. However, recently, it is reported that by mixing a monomer in a liquid crystal and irradiating the liquid crystal with ultraviolet rays in the temperature range developing blue phase liquid crystal, it is possible to obtain a polymer-stabilized blue phase liquid crystal in which the temperature range developing the blue phase liquid crystal is expanded to at least 60° C. (for example, refer to Non-Patent Document 2).
By the way, in an optical head device for writing and/or reading (hereinafter referred to as “writing and/or reading”) an information to/from an information recording plane of an optical recording medium such as an optical disk such as CD or DVD, or a magneto-optical disk, light emitted from a laser diode is converged on an information recording plane of the optical disk through an objective lens, and reflected to be returning light, and the returning light is guided through a beam splitter to a photo-receiving element as a photodetector. Here, as the beam splitter, by employing e.g. a hologram beam splitter as a sort of diffraction element, it is possible to deflect the propagating direction of light by diffraction to guide the light to the photodetector, whereby it is possible to realize downsizing of optical head device.
Further, a DVD/CD compatible optical head device has been put into practical use, which is an optical head device employing a laser diode for DVD and a laser diode for CD and capable of writing and/or reading informations to/from optical disks of DVD and CD as optical information mediums having different standards.
Here, in order to realize downsizing of such an optical head device and increase of light-utilization efficiency, Patent Document 1 describes a polarizing diffraction element having wavelength selectivity, wherein a concave-convex portion of an rectangular diffraction element made of a polymer liquid crystal whose alignment direction is uniform, is fabricated so as to produce a phase difference of a natural number times of a wavelength for CD for extraordinarily polarized light having the wavelength for CD, and so as to produce no phase difference for ordinarily polarized light, whereby the polarizing diffraction element transmits ordinarily polarized light in a 650 nm wavelength band for DVD, diffracts extraordinarily polarized light in the wavelength band is diffracted, and transmits incident light of 790 nm wavelength band for CD regardless of its polarization state.
Non-Patent Document 1: Chandrasekhar, “Liquid Crystals”, Second edition, Cambridge University Press, Chap. 4 FIG. 4. 1,6)
Non-Patent Document 2: “Nature Materials”, vol. 1, no. 1, Macmillan Press, September 2002, p. 64-p. 68
Patent Document 1: JP-A-2001-174614